This also applies to things people say. For some reason, I remember remarks that people made decades ago. And it's not that what they said was so striking or perceptive; some are downright boring remarks. But they stayed with me and I think of them often. There seems to be no rhyme or reason governing what remains in our minds, and what passes through, virtually unnoticed.
(Don't get me wrong. Some of the memories that stay with us forever are important. But it seems our brains store all sorts of things, including stuff that is meaningless.)
Of course, I'm going to bring this around to writing. The odd thing is that these memories -- both significant and insignificant -- are finding a home in the novels I write. I'm not aware that I'm doing this as I write a scene. But later when I read it, I'll realize that I've recreated one of these fortified memories in the book. The look of a room, the feeling caused by an offhand remark, an odd phrasing someone once used -- all these memories that stuck with me throughout my life are being released, one by one, into my novels. It's quite nice, actually. They finally found a good home.
Do you have strong memories of an event or a day that didn't seem to matter much when it happened, but nonetheless stayed with you all these years? It's such a quirky thing.